1. Field of the Invention
This invention is generally directed to chemical and biochemical quantitative analysis, and more specifically concerns an optical fiber sensor for measuring multiple parameters such as oxygen, carbon dioxide, and pH of a fluid or gaseous mixture.
2. Description of Related Art
Fiber-optic based devices for measuring concentrations of pH, oxygen and carbon dioxide have found numerous applications in the medical, chemical and environmental fields. Optical fiber sensors have also now been developed for taking in vivo, intravascular measurements of blood analytes, such as pH, oxygen and carbon dioxide. Many such sensors rely on the phenomenon of dye fluorescence in response exposure to an excitation wavelength of light as a means for measuring the presence of analyte in a liquid or gaseous mixture. Fluorescence dye indicators have been widely used for such devices due to the high sensitivity that can be achieved. Systems and instruments implementing fluorescence techniques typically utilize an encapsulated fluorescent dye whose fluorescence emissions are affected by the presence of the analyte of interest. The fluorescent dye can be placed within a semi-permeable matrix made from a polymer or similar substance. A light source with appropriate filtering system provides a selected wavelength of light which propagates down the optical fiber and excites the dye. The fluorescence signal, induced by the excitation energy, can also return via the same optical fiber, to be measured by a photodetector. The intensity of the fluorescence of the dye, which is a function of the analyte level in the sample, can be transduced into a measure of the concentration of the analyte of interest.
A fluorescent sensor typically utilizes light in one wavelength region to excite the fluorescent indicator dye to emit light of a different wavelength. Such a sensor may for example utilize a single dye that exists in an acid form and a base form, each with a different excitation wavelength to measure pH.
The concentration of carbon dioxide in a solution can be determined by an optical sensor by measuring the pH of a solution of bicarbonate in equilibrium with the carbon dioxide in the solution. The bicarbonate and carbon dioxide form a pH buffer system in which the hydrogen ion concentration generally varies with the carbon dioxide concentration. The pH or carbon dioxide content of a solution may, for example, be measured with a fiber optic sensor utilizing fluorescein as a fluorescence indicator enclosed in a silicone matrix at the end of an optical fiber. Another type of fluorescence indicator which has been used is hydroxypyrenetrisulfonic acid (HPTS).
Techniques implementing fluorescence quenching for measuring the partial pressure of oxygen have been developed which utilize an encapsulated oxygen-quenchable fluorescence dye that is placed within a gas permeable matrix usually made from a polymer or similar substance. The intensity of the fluorescence of the dye, which is a function of the oxygen level in the sample, can be transduced into a partial pressure of oxygen.
Relatively bulky multiple optical fiber sensor probes having separate optical fiber sensing elements for each analyte have been developed, but are complex and difficult to manufacture. Although an optical fiber fluorescent dye based sensor for sensing both oxygen and CO.sub.2 has been developed, which uses separate layers containing different dye-polymers for sensing different analytes, these sensors can also be difficult to manufacture, and may cause cross-interference in one or more of the indicator layers. There therefore remains a need for an optical fiber sensor including multiple dye indicators in a single matrix layer, for sensing multiple analytes.
While many optical fiber based sensor elements have been developed, there are also inherent problems commonly associated with them that are detrimental to the accuracy of the measurements. For example, it is sometimes difficult to immobilize the fluorescent dye in a gas permeable matrix because of a chemical incompatibility between the dye and matrix. Many of the more widely used fluorescent dyes are polynuclear aromatic compounds which have low solubility in organic materials. As a result, the fluorescent dyes have a tendency to leach through the permeable matrix into the solution or gas mixture that is being tested.
Various approaches for creating an operable sensor element include absorbing the dye on inorganic or organic solid supports, dispersing the dye in the matrix by way of organic solvents, and covalently bonding the dye on porous glass. Many of these techniques still have serious drawbacks if the dye is chemically incompatible with the polymer matrix. Such dyes can have a tendency to leach out, particularly when in contact with a sample that includes a substance that has similar properties as the dye polymer matrix. Unfortunately, such substances include blood proteins and many organic solvents, which are often present in the samples being tested. As a result of the leaching of the dye during use, the sensing element may have to be continuously replaced to ensure the accuracy of analyte measurements. Moreover, dye molecules that are free to move within a polymer matrix may also tend to agglomerate, which results in changes in their fluorescent properties.
One approach to construction of an optical sensor has involved the application of sensing material directly to the tip of the optical fiber, or the attachment of a dye filled porous glass to the tip of the optical fiber, by an adhesive. Another approach has involved the attachment of a sleeve which contains the dye indicator sensing material immobilized in a hydrophilic polymeric matrix, such as by entrapment in the matrix or by ionic interactions with the matrix, over the tip of the optical fiber. However, such sensors tend to eventually allow the indicator dye to leach out over extended time periods. Such leaching of the indicator dye results in increasingly inaccurate blood pH measurements.
Covalently bonding a dye indicator to an optical fiber core or to a polymer matrix secured over the core can reduce indicator leaching in such optical fiber sensors. In one approach, for example, the dye can be covalently bonded to the polymer, and the cross-linked polymer can in turn be covalently attached to the fiber. However, the dye loading of the carrier polymer is controlled by the fixed number of sites on the carrier polymer, and commonly only one type of functional group is available for dye attachment and crosslinking, even where the carrier polymer includes multiple dye bonding sites spaced to avoid physical cross-interference.
There thus remains a need for an optical fiber sensor which provides covalent linkages between the dye and matrix, and between the matrix and the optical fiber, to prevent leaching of the indicator material during periods of extended use of the sensor. It would also be desirable to provide such a dye matrix system to be formed from a copolymer to control not only the concentration of dye in the final sensor matrix, but also to control the relative proportions of different dyes in the final matrix. It would be desirable to provide such a copolymer system with different types of functional sites for bonding different dye indicators, and for cross-linking, which would allow the number of sites present on the carrier polymer to be altered depending upon the sensor requirements.